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1998 Honda Odyessy (4 cylinder) keeps setting code p0304.

Plugs are good, wires and cleaned fuel system. Are there some EGR passages that need to be cleaned?
Also, it has good fuel pressure.

Yes, there are EGR ports that are known for clogging. Remove the cover on the top of the intake manifold runners (located under the fuel rail) and clean the EGR port holes in the intake runners.

Possibly valve adjustment?

304 indicates #4cylinder misfire. this could in fact be #4 issue or i've seen bad crank sensors do this. unfortunately it's a matter of elimination to solve this. i would replace # 4 plug and see if it goes away. you said plugs were good. have they been replaced? the wires are good. did you check for spark jump. we use the old spark tester that goes in line. if it's a dull flash replace we check the others and compare. compression test as well as a compression while running. if all that is well we swap injectors and see if the code moves. if all tests prove good and code remains #4 replace the distributor. i may be wrong, but i believe the crank sensor is in the distributor. if not replace the censor. i'm at home and can't open the repair program. again i'm at home but i don't think there are valve adjustments and a quick way to see if the egr injector is plugged is remove the valve and spark plugs n that bank. shove a blow gun n the intake port and stuff a fag around it the best you can or seal it somehow. it doesn't have to be air tight. blow air into the port and see if there is air coming out of the cylinders. if all, no restriction. if all but #4 there's your problem. now before you tear off the intake a little trick here. put the plugs back in all but #4. spray carbon buster into the port and blow it in with the blow gun. really lay it in and blow it around. i would suggest using the whole can. we have nylon tuning we use for a/c vent service and we attach it to the can and really get it deep into the port. let it soak for a while. i don't mean a few minutes. over night if possible. install blow gun into the intake port and really seal it the best you can. blast away with as much air as you can. hopefully you'll hear air coming out of the cylinder or i've heard a pop on fords and all is well. clean it until your satisfied. here's where it gets a bit off the wall. we start the engine a few times with the plug out of the effected cylinder. between starts we spray brake cleaner into the cylinder to clean it out. the carbon that cleared out may be solid enough to cause damage. unplug wire from cap. i've seen a flame thrower.

Check the valve clearance

Thanks for all your input customer didn't have time to bring it back, thanks everyone.

Hopefully the customer will bring it back and let you discover what the problem is. It is not nice when we can't have vehicle long enough to do the job but its part of business I guess.